1. Field of the Invention
This invention relates generally to rotating mirror light scanners and, more particularly, to rotating mirror light scanners used in remote vehicle emission analyzers.
2. Description of the Background
In view of the increasing concern with the degradation of the environment caused by motor vehicle emissions, there have been developed several systems for remotely analyzing the exhaust emissions of a motor vehicle as that vehicle drives through or passes a remote analyzing station. Typically, a light beam is directed across the roadway so that the beam of light passes through the plume of the vehicle exhaust and falls on several detector units that detect the levels of various gaseous components of the vehicle exhaust. In a known system only one light beam is used to intercept the vehicle exhaust plume, and that beam is scanned using an optical assembly onto a number of detector units arranged generally in the same housing and usually on the opposite side of the road from the light beam emitting assembly. The single beam can be caused to impinge successively on the several detectors using a flying spot scanner comprised of a multifaceted, rotating mirror. The beam, after having passed through the vehicle exhaust plume, is directed by a primary mirror onto the rotating mirror and as the mirror undergoes its rotational translation the beam is caused to be scanned onto a number of secondary mirrors that reflect their light beams onto respective detector units. The detector units are arranged side-by-side and at the same horizontal level, and the secondary mirrors are mounted around 8-10 inches from the rotating mirror.
This system works well in principle, however, it is difficult to set up because all of the components must be accurately aligned optically, in order for the beam to fall fully on each detector. In addition, optical misalignments and mechanical vibrations cause noise in the output signals of the detectors. Also, as in any optical system, the intensity of the light is diminished by action of the various optical elements so that the detector output signals may be of such a low level as to be susceptible to electrical noise as well.
Furthermore, the more times light is reflected in a design the more susceptible that design is to mechanical vibrations. Because the secondary mirrors are located 8 inches from the spinning mirror, the light at that point in the system is diluted, diffused, and susceptible to mechanical vibration. Also, because of the necessary optical alignment the secondary mirrors are partially blocked by the detectors and the mounting arrangement.
Light focused by the secondary mirrors onto the respective detector elements is non-homogeneous. This exacerbates signal noise level when slight movements in the system occur. Furthermore, as in any optical system, very precise alignment is required for successful operation.
Representative of previously proposed systems for use in scanning a light beam onto a number of detectors are the systems shown in U.S. Pat. Nos. 5,210,702 and 5,343,043.